Polyethylene or random polypropylene pipes of this kind are used mainly for the distribution of gas and drinking water in particular in civil and industrial installations.
Polyethylene or random polypropylene pipes are made in continuous extrusion lines.
In this production line, the material in the plastic state is fed by a screw which rotates inside a cylinder and forces the material through a die of suitable shape and size giving the material the required circular shape.
The pipe production installation, that is to say, the extrusion line, normally comprises a sequence of machines, each with specific functions.
One of these machines, located at the end of the extrusion line, is a cutting device designed to cut the pipe into lengths of precise, predetermined size.
The cutting device is usually an automatic machine tool, known as “cutter”, capable of cutting the pipe as it moves in a straight line at a constant speed.
The cutter comprises a mechanical unit, known as cutting unit, which imparts the working motion to the cutting tool.
The cutting unit is installed on a carriage which moves in synchrony with the pipe during the cutting operation.
The carriage has a pair of vices designed to clamp the pipe in place, During the cutting operation, these vices clamp the pipe upstream and downstream of the separating plane (known as the cutting plane) of the length of extruded pipe.
In the relevant field of cutting polyethylene or random polypropylene pipes having large diameters and thick walls, cutters equipped with sawtooth cutting tools may be used which perform cutting by removal of material.
As is known, removal of material produces swarf which must be extracted because it obstructs the cutting zone, preventing the cutter from operating properly.
Moreover, the swarf produced during cutting contaminates the working environment and may damage other devices forming part of the cutting unit and may make it difficult or impossible to perform subsequent operations such as sealing the ends of the pipes to each other or to end fitting joints.
A further disadvantage of cutters of this type is that they are noisy and produce vibrations which are transmitted to the working parts and frame of the cutter itself.
Another type of cutter for pipes made of thermoplastic material comprises circular disk knives which rotate freely about their axes.
The disk is driven in rotation about the axis of the pipe.
This type of cutter overcomes the above mentioned disadvantage of swarf because cutting is accomplished by separation,
It has been found, however, that the larger the diameter and the thicker the walls of the pipe to be cut, the higher the torque required to drive the disk in rotation about the axis of the pipe.
Thus, for a pipe with a particularly large diameter and a very thick wall the torque required to drive the disk in rotation about the axis of the pipe is extremely high.
It has also been found that the forces generated by contact between the disk knife and the pipe walls during cutting are particularly high and cause rapid disk wear.
A need that is therefore felt particularly strongly by operators in this sector is that for a cutting machine which allows cutting pipes made of thermoplastic material, in particular, semi-crystalline thermoplastic material such as polyethylene or random polypropylene and which does not suffer from the above mentioned disadvantages.